1. Field of the Invention
The present invention relates generally to wind tunnels. More particularly, the present invention relates to closed circuit wind tunnels and the use of a plate-fin heat exchanger in such wind tunnels.
2. Description of the Prior Art
Wind tunnels help researchers simulate the forces acting on an object moving through air. To obtain useful results, the conditions in the wind tunnel should closely match the conditions the object will encounter in actual service.
Flow generation means in the form of a fan generally drives the air flow in a wind tunnel to create the tunnel flowstream. Unfortunately, much of the energy supplied by the fan is eventually converted into heat in the tunnel flowstream. This means that air flow in a high-powered wind tunnel must be cooled to order to maintain functional testing conditions.
Since the inception of heat exchangers in wind tunnels (circa 1940), wind tunnels have used a xe2x80x9cfin-tubexe2x80x9d style, water-cooled radiator type of heat exchanger. Such heat exchanger transfers the heat energy from the flowstream to a coolant. The heated coolant is then pumped out of the heat exchanger, cooled by external means such as a cooling tower, and then recirculated to the heat exchanger. This xe2x80x9cfin-tubexe2x80x9d type of heat exchanger consists of coolant-carrying tubes that cross back and forth across the air flow passage. These tubes include attached fins that help collect the flowstream heat and transfer it to the coolant. Several problems exist with this type of heat exchanger for wind tunnel applications. For example a xe2x80x9cfin-tubexe2x80x9d type of heat exchanger presents a very large resistance to the flowstream. The resistance increases the power needed to drive the wind tunnel, which in turn increases the temperature of the flow even more. To reduce this resistance, many wind tunnels increase the size of the heat exchanger in the cross-flowstream direction. This in turn requires the expansion of the wind tunnel duct cross-section to house the larger heat exchanger. The transition from a smaller duct upstream of the heat exchanger to the larger duct required to house the heat exchanger often requires the use of a wide angle diffuser which significantly increases the risk of flow separation, turbulence, and angularity problems.
Further, for large wind tunnel applications, multiple modules or cores of xe2x80x9cfin-tubexe2x80x9d type heat exchangers are commonly supported as an array in an xe2x80x9cegg cratexe2x80x9d type structure. Thus, large coolant feed and return pipes must be routed within this xe2x80x9cegg cratexe2x80x9d to each of the heat exchanger modules. These support structures and piping take up considerable space, thereby resulting in a significant increase in flowstream resistance and misdirection of the air flow.
Still further, the flow around the cross-stream tubes in a xe2x80x9cfin-tubexe2x80x9d heat exchanger produces unsteady turbulent flow characteristics which in turn causes dynamic forces on the tube. These forces may induce tube vibration due to the low natural frequency of the slender, long span coolant tube which in turn may lead to undesirable noise and high stress or fatigue which often results in tube failure or leaks. The cross-stream tubes also cause unsteadiness and separation in the flowsteam. These effects degrade the quality of any experimental results. Still further, because the fins in the heat exchanger are press fit onto the tubes, the unsteady flow and resulting vibrations over time can cause the fins to separate or lose their grip on the tube. This results in a degradation of the heat transfer effectiveness of the heat exchanger.
Accordingly, there is a need in the art for a wind tunnel with an improved heat exchanger structure which solves the problems and overcomes the limitations of the prior art.
In contrast to the prior art, the present invention relates to a wind tunnel with an improved heat exchanger structure that overcomes the problems and limitations of the prior art. Specifically, the wind tunnel and heat exchanger combination of the present invention provides for a more compact and efficient heat exchanger capability, reduces turbulence in the flowstream and minimizes the flowstream blockage or resistance resulting from the heat exchanger.
More specifically, the present invention includes a wind tunnel with a xe2x80x9cplate-finxe2x80x9d heat exchanger comprised of a plurality of heat transfer fins separated by a plurality of parting sheets. The heat exchanger fins and parting sheets create a plurality of passageways or channels for passage of the air flowstream and the cooling fluid.
xe2x80x9cPlate-finxe2x80x9d heat exchangers have particular application to wind tunnels and offer significant advantages in flow quality over wind tunnel designs of the prior art. First, plate-fin heat exchangers eliminate the turbulence generated by flow unsteadiness and separation around the tubes. This improves the flowstream through the test section and allows for better and more accurate measurements. Lower turbulence levels also enhance the acoustic characteristics of the tunnel, which can be very critical for certain types of testing. A second advantage of the plate-fin heat exchanger over the prior art is its flow straightening effect. The plate-fin modules are made of many small cell, long length flow passages. These passages are similar to the honeycomb flow straighteners typically installed in wind tunnels. A third advantage in flow quality of plate-fin heat exchangers is that their high efficiency allows designers to reduce the size of, or even eliminate the need for, a wide angle diffuser which is often required when using xe2x80x9cfin-tubexe2x80x9d heat exchangers.
Besides the advantages in flow quality, plate-fin heat exchangers also offer significant economic advantages. These advantages include: i) reduced capital costs because the tunnel shell can be shorter and the cross sectional size of the tunnel can be smaller; ii) reduced operational costs due to higher thermal and aerodynamic efficiencies; iii) lower piping and support structure costs because the design requires fewer pipes inside the wind tunnel; iv) simpler maintenance and removal because the coolant piping is accessible from outside the tunnel; v) lower cost of other flow conditioning equipment because the plate-fin heat exchanger improves, rather than degrades, flow quality; vi) fewer leaks and ruptures in the coolant tubing because the plate-fin modules vibrate less and are structurally more robust; and vii) improved maintenance in thermal efficiency over time resulting from a reduction of vibration induced fin degradation.
Accordingly, it is an object of the present invention to provide a closed circuit wind tunnel with an improved heat exchanger.
Another object of the present invention is to provide a wind tunnel with a heat exchanger which minimizes turbulence in the flowstream.
A further object of the present invention is to provide a wind tunnel with a heat exchanger which is highly efficient and economical.
A still further object of the present invention is to provide a closed circuit wind tunnel with a plate-fin type heat exchanger.
These and other objects of the present invention will become apparent with reference to the drawings, the description of the preferred embodiment and the appended claims.